1. Field of the Invention
This invention relates to the polymerization of vinyl acetate to from poly(vinyl acetate), and the production therefrom of poly(vinyl alcohol). More particularly, this invention relates to such a process for the production of poly(vinyl acetate) and poly(vinyl alcohol) of high molecular weight.
2. Prior Art
Poly(vinyl alcohol) is useful for many purposes. For example, poly(vinyl alcohol) of especially high viscosity is used as thickening agent, protective colloid and the like in which high viscosity solutions having low solid content are desired. Moreover, poly(vinyl alcohol) is used in textile and paper sizing, in adhesives, as an emulsion polymerization aid, and as an intermediate in the production of poly(vinyl butyral), the adhesive interlayer in laminated safety glass. Furthermore, poly(vinyl alcohol) is used for textile fiber after water-insolubilization.
High molecular weight poly(vinyl alcohol) is a high-melting, high-strength material which is suitable for a wide variety of applications including tire cord, housing materials, automotive plastics, super strength fibers, and the like. The excellent adhesive properties of poly(vinyl alcohol) render it potentially an outstanding tire cord material.
Poly(vinyl alcohol) is conventionally produced in a two-step process. In the first step of the process, vinyl acetate is polymerized to produce poly(vinyl acetate). In the second step, the poly(vinyl acetate) is subjected to alcoholysis (methanolysis or ethanolysis) in order to convert the poly(vinyl acetate) to poly(vinyl alcohol).
Several methods have been proposed for the manufacture of poly(vinyl alcohol) of relatively high molecular weight. For example, Canadian Pat. No. 663,529 discloses an emulsification process for producing poly(vinyl alcohol) in which a vinyl acetate emulsion is formed, and polymerization is initiated by irradiating the emulsion in the absence of oxygen with ionizing radiation to a total radiation dose in the range of 1.20.times.10.sup.4 roentgens to 2.32.times.10.sup.4 roentgens at a temperature of from 50.degree. C. to -15.degree. C. to form an essentially linear poly(vinyl acetate) having an intrinsic viscosity of 1.7 to 3.2 dL/g.
Based upon numerical calculation, the poly(vinyl acetate) of Canadian Pat. No. 663,529 has a viscosity average molecular weight ranging from about 640,00 to about 1,500,000. Alcoholysis of the poly(vinyl acetate) of Canadian Patent No. 663,529 would produce poly(vinyl alcohol) having a viscosity average molecular weight ranging from about 327,000 to about 765,000.
A similar radiation initiated bulk polymerization procedure is described in United Kingdom Pat. No. 900,571. In this patent, vinyl acetate is polymerized by subjecting it to ionizing radiation in the absence of oxygen or other reactive substances at a temperature below the temperature at which thermal polymerization of the vinyl acetate occurs. The resulting poly(vinyl acetate) is hydrolyzed to produce the desired poly(vinyl alcohol). United Kingdom Pat. No. 900,571 discloses that the poly(vinyl alcohol) produced by the process of that patent has the molecular structure: ##STR1## where n is the average degree of polymerization with n being 4,700. The poly(vinyl alcohol) of United Kingdom Pat. No. 900,571 has a viscosity average molecular weight of about 200,000, and the corresponding poly(vinyl acetate) of United Kingdom Pat. No. 900,571 has a viscosity average molecular weight of about 404,000.
Some investigators indicate that production of essentially linear poly(vinyl acetate) requires a polymerization temperature below about -15.degree. C. Atkinson, et al., European Polymer Journal, Vol. 15. pp 21-26 (1979); Hobbs, et al., Journal of Polymer Science, Vol. XXII, pp. 123-135, 124 (1956); Burnett, et al., Journal of Polymer Science, Vol. XVI, pp. 31-44, 39 (1955). Hobbs, et al. disclose at page 125 the preparation of an essentially linear poly(vinyl acetate) having an intrinsic viscosity of 3.11. In order to produce this polymer, Hobbs, et al. employed azobisisobutyronitrile as an initiator at a concentration of 2.33.times.10.sup.-4 moles per liter and allowed the polymerization to continue for 37.5 hours at a temperature of -19.degree. C.
Burnett, et al. found that a polymerization temperature of -25.degree. C. to -38.degree. C. could be employed in order to obtain essentially linear poly(vinyl acetate). However, Burnett, et al. noted that polymers prepared at -38.degree. C. had essentially the same molecular shape as those prepared at -25.degree. C. A contrary view is that the polymerization temperature is irrelevant and that essentially linear poly(vinyl acetate) may be obtained as long as the polymerization is not allowed to proceed to high conversions of the monomeric vinyl acetate. Matsumoto, et al., Journal of Polymer Science, Vol. XLVI pp. 520-523 (1960).
U.S. Pat. No. 4,463,138 discloses that poly(vinyl acetate) can be produced by free radical bulk polymerization of vinyl acetate with controlled low initiator concentrations and with ultraviolet radiation to activate the initiator, employing a polymerization temperature of between about -25.degree. C. and about -45.degree. C. for a period of time between about 24 hours and 120 hours. This patent discloses that the poly(vinyl alcohol) can be prepared by conventional alcoholysis of the poly(vinyl acetate). Poly(vinyl alcohol) prepared in accordance with the process has an intrinsic viscosity greater than about 5 dL/g.
United Kingdom Pat. No. 2,105,354 discloses a process in which ethylene and vinyl acetate are continuously copolymerized in a solvent in the presence of a radical initiator whose half life measured at 60.degree. C. is not longer than 2 hours, such as 2,2'-azobis-(4-methoxy-2,4-dimethylvaleronitrile), at a temperature in excess of 50.degree. C. The patent states that use of such inhibitors reduces gel formation.